Aqueous-developable dry-film photoresists are photopolymerizable compositions used in the manufacture of printed circuit boards. Such photoresists are generally made by applying the solvated resist material to a carrier, such as a transparent polyester film, and then evaporating the solvent to produce the dry film. In typical use, a dry-film photoresist is applied to a copper-clad substrate along with the carrier, exposed in certain areas through the carrier to actinic radiation that will cure the film, and then washed with an alkaline aqueous solution to remove the unexposed film from the copper. The exposed copper surface can then be removed in etching solutions leaving the protected area under the cured photoresist to form the electrical circuit.
Manufacturing operations such as alkaline etching and gold plating typically involve alkaline conditions that cause particular problems, such as loss of adhesion of the cured photoresist. Accordingly, it is important that the cured photoresist be tough enough to withstand such procedures.
It is also advantageous to have a photoresist that in unexposed areas is readily washed from the substrate in order to achieve as rapid a development time as possible. To facilitate easy removal of uncured photoresist, one of the ingredients in photoresists is a carboxyl-group containing film-forming polymeric binder.
However, a delicate balance of ingredients must be maintained in photoresist formulations. Materials that advantageously affect toughness of the cured resist might adversely affect exposure time or development time or might cause the cured photoresist to become brittle.